Biology: The Study of Life

What Does It Mean to Say That Something is Alive?

Biology is the scientific study of life, and all living organisms share five fundamental characteristics that distinguish them from non-living matter:

• Cells: All organisms are composed of membrane-bound cells, which are the basic units of life.

• Replication: All organisms are capable of reproduction, ensuring the continuation of their species.

• Information: Organisms process hereditary information encoded in genes and respond to environmental information.

• Energy: All organisms acquire and use energy to sustain life processes.

• Evolution: Populations of organisms are continually evolving, adapting to their environments over time.

Theories in Biology

Biological science is built upon several foundational theories:

• Cell Theory: All organisms are made of cells, and all cells come from preexisting cells.

• Theory of Evolution by Natural Selection: Explains how organisms are related and how they change over time.

• Chromosome Theory of Inheritance: Describes how hereditary information is transmitted from one generation to the next.

Life is Cellular and Replicates through Cell Division

Discovery of Cells

The cell theory originated from the work of early microscopists:

• Robert Hooke (1665): Observed cork tissue and coined the term "cells."

• Anton van Leeuwenhoek: Observed single-celled organisms, referred to as "animalcules."

Cells are highly organized compartments separated from their environment by a membrane barrier.

Cell Theory and Spontaneous Generation

Cell theory challenged the idea of spontaneous generation:

• All-cells-from-cells hypothesis: Cells are produced when pre-existing cells grow and divide.

• Spontaneous generation hypothesis: The belief that organisms could arise spontaneously under certain conditions.

Louis Pasteur’s Experiment

Pasteur tested whether cells arise from pre-existing cells or by spontaneous generation using nutrient broth in flasks:

• Only flasks exposed to pre-existing cells developed new cells, supporting the all-cells-from-cells hypothesis.

Terminology in Science

• Theory: Explanation for broad patterns in nature, supported by evidence.

• Hypothesis: Testable statement explaining an observation.

• Experiment: Method to test the effect of a single factor.

• Prediction: Observable result expected if the hypothesis is valid.

Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

Hereditary information is encoded in genes located on chromosomes, which are molecules of deoxyribonucleic acid (DNA). Genes are segments of DNA that code for cell products.

Structure of DNA

DNA is a double helix composed of four building blocks: A (adenine), T (thymine), C (cytosine), and G (guanine). The sequence of these bases encodes genetic information.

Base Pairing and DNA Replication

• A pairs with T, and C pairs with G.

• This pairing allows DNA to be copied accurately, preserving genetic information.

The Central Dogma

The central dogma describes the flow of genetic information in cells:

• DNA codes for RNA, which codes for proteins.

• Proteins determine physical traits and carry out cellular functions.

RNA and Proteins

• RNA: Carries out specialized functions; messenger RNA (mRNA) is read to make proteins.

• Proteins: Crucial for cell structure and chemical reactions.

Genetic Variation and Mutation

• DNA is copied to pass genetic information to offspring.

• Mutations (changes in DNA sequence) can alter proteins and lead to heritable variation, driving diversity.

Energy and Metabolism

• Cells require energy for chemical reactions.

• Organisms acquire energy in the form of adenosine triphosphate (ATP) and building blocks for macromolecules.

Energy Acquisition in Organisms

• Plants and bacteria can produce sugar using sunlight, then use or store it as ATP.

• Organisms may also absorb molecules from their environment as food.

Life Evolves

Evolution and Natural Selection

Evolution is the change in characteristics of populations over time. Darwin and Wallace proposed:

• Species are related by common ancestry.

• Characteristics can be modified from generation to generation (descent with modification).

Population and Natural Selection

• Population: Group of individuals of the same species living in the same area.

• Natural selection requires heritable variation and differential reproductive success.

• Traits that increase fitness become more common; speciation occurs when populations diverge.

Fitness and Adaptation

• Fitness: Ability to produce surviving offspring.

• Adaptation: Trait that increases fitness in a particular environment.

• Example: Finches with small, pointed beaks had higher fitness when small seeds were abundant.

The Tree of Life Depicts Evolutionary History

Phylogeny and Genetic Variation

The tree of life describes genealogical relationships among species, with a single ancestral species at its base. Phylogeny is the actual genealogical relationship among organisms.

Analyzing Genetic Variation

• Biologists compare DNA and RNA sequences to determine relatedness.

• Fewer sequence differences indicate closer relationships.

Phylogenetic Tree of Life

• Phylogenetic trees show relationships between species.

• Branches sharing recent common ancestors represent closely related species.

Major Groups of Organisms

• Eukaryotes: Have a nucleus (Eukarya).

• Prokaryotes: Lack a nucleus (Bacteria and Archaea).

Taxonomy and Classification

Taxonomy

• Taxonomy is the effort to name and classify organisms.

• A taxon is a named group.

• Domain: Highest taxonomic level; three domains: Bacteria, Archaea, Eukarya.

• Phylum: Major lineage within a domain.

Linnaeus’ System and Nomenclature

• Each organism is given a unique two-part scientific name: genus and species.

• Genus names are capitalized; species names are not.

• Scientific names are italicized (e.g., Homo sapiens).

Doing Biology: The Nature of Science

Scientific Method and Hypothesis Testing

• Science involves formulating hypotheses and testing them with evidence.

• Hypothesis testing involves stating a hypothesis and predictions, then designing studies to test them.

Example: Why Do Giraffes Have Long Necks?

• Food competition hypothesis: Long necks evolved to reach food high in trees.

• Tested predictions: neck length variability, heritability, feeding behavior.

• Alternative hypothesis: Sexual competition—longer-necked males win more fights and father more offspring.

Experimental Design: How Do Ants Navigate?

• Experiments test the effect of a single factor.

• Pedometer hypothesis: Ants track steps and stride length to navigate.

• Null hypothesis: Stride length and step number have no effect.

• Manipulated ant leg length; results supported pedometer hypothesis.

Characteristics of Good Experimental Design

• Include a control group.

• Keep experimental conditions constant.

• Repeat tests and use large sample sizes.

Summary Table: Fundamental Characteristics of Life

Additional info: Expanded explanations and examples were added for clarity and completeness, as per academic context.